The present invention relates, in general, to the field of laser rangefinding and speed measurement instruments. More particularly, the present invention relates to laser source modification techniques for a laser-based rangefinding or speed measurement instrument enabling increased range, better defined beam pattern and improved accuracy while remaining within applicable eye-safety limits.
Laser rangefinders, such as those designed and produced by Laser Technology, Inc., Centennial, Colo., assignee of the present invention, operate to calculate distance by measuring the time of flight of very short pulses of infrared light. That is, a measurement is made as to the time it takes one or more laser pulses to travel to a target and back with a precision time base. With knowledge of the constant speed of light, the distance the laser pulses have traveled can then be calculated.
In order to increase accuracy, such laser rangefinders are designed to process multiple pulses in a single measurement period, with target acquisition times typically ranging from 0.3 to 0.7 seconds. Sophisticated accuracy validation algorithms are then utilized to ensure reliable distance measurements and eliminate spurious signals due to noise and other factors.
Laser Technology, Inc. has pioneered and developed the design and measurement functionality found in some of the most popular lines of rangefinders and speed measurement instruments currently available on the market. Representative of its proprietary technology is that disclosed in U.S. Pat. Nos. 5,574,552; 5,612,779; 5,652,651; 5,703,678; 5,880,821; 5,926,260; 6,057,910; 6,226,077 and 6,445,444, the disclosures of which are herein specifically incorporated by this reference in their entirety.
Laser-based speed and rangefinding instruments have to adhere to strict eye-safety standards and consumer devices in particular have to adhere to U.S. Food and Drug Administration Title 21 and International Electrotechnical Commission (IEC) 60825 Class 1 standards. These regulations define a 514 nanojoules per pulse energy maximum. In reality, this figure must then be further reduced and divided by the fourth root of the number of pulses emitted in the applicable time base, which is typically just over ten seconds, thereby reducing the number further.
In addition to this are the inevitable correction factors which must be accounted for such as αmin (alpha min; the angular subtense of a source below which the source can be effectively considered as a point source), beam exit size and the like. All these factors go into a determination of the limit of the nanojoules per pulse that are allowed in order for the instrument to remain within the applicable Class 1 limits.
High power pulsed laser diodes (PLDs) for rangefinding, speed monitoring and other applications are available, for example, at wavelengths of 905 nm. At 905 nm, the Class 1 limit is what is measured at the output of the instrument and not what the PLD itself can put out, which can be greater than the Class 1 eye-safety limit.
Rangefinding instruments may also be based on those PLDs having a wavelength centered at 1550 nm and they can be operated at high power levels while still remaining in an eye-safe range. At 1550 nm, the PLDs are more expensive than those at 905 nm (and only about ⅓rd as efficient) and not enough energy can be output to reach the Class 1 limit with currently available diodes. As they are inherently eye-safe, more energy can be output, resulting in increased range, if the source can emit the requisite energy. Most common night-vision googles cannot detect this wavelength and they are often employed in military rangefinding instruments.